This invention relates generally to surface acoustic wave (SAW) devices, and more particularly, to spectrum analyzers implemented in SAW technology. By way of brief background, SAW devices employ substrates of a piezoelectric material, across which elastic surface waves are propagated between sets of electroacoustic transducers disposed on the substrate surface. The devices employ so-called Rayleigh waves, which can be propagated along a free surface of a solid, and have an amplitude of displacement that is largest right at the substrate surface. In a piezoelectric material, deformations produced by such waves induce local electric fields, which are propagated with the acoustic waves and extend into space above the surface of the material. These electric fields will interact with electrodes disposed on the surface of the material, to serve as electrical input and output transducers for the surface acoustic wave device.
Although most SAW devices are "in-line" devices employing a single propagation direction, SAW technology can also be applied to diffraction-effect devices, such as spectrum analyzers. A SAW spectrum analyzer is disclosed in a copending application of Robert E. Brooks, Ser. No. 529,066, filed on Sept. 2, 1983, and entitled "Signal Processing System and Method." The Brooks application describes the basis on which a SAW spectrum analyzer operates. In essence, the principle of operation is closely analogous to that of an optical diffraction grating. When a collimated beam of light is incident on a plane grating, the scattered light is dispersed into monochromatic waves propagating at angles dependent on their wavelength. If the scattered waves are imaged to points or lines by a focusing lens, a number of diffraction orders will be seen. In each order except the zero order, the light is dispersed into its spectral components. This basic property is used in the optical spectrograph, in which the grating is curved to eliminate the need for a focusing lens, and is "blazed" to provide a multiplicity of reflective scattering strips. By this means, the grating scatters light only in a single diffraction order, and no energy is lost to the unused orders.
The SAW counterpart of the optical spectrograph is closely analogous to a curved and blazed diffraction grating. As in the optical spectrograph, the device is constructed so that almost all of the energy is confined to the first order. Basically, the SAW device comprises a curved input transducer array having a large number of wideband interdigital transducers connected in parallel, and an array of output transducers. Each input transducer is so small that it behaves very much like a point source of energy, which can be considered to radiate circular wavefronts if the anisotropic nature of most SAW substrate materials is neglected.
The curvature of the input transducer array causes the energy from the array to focus at a focal point located at a predetermined focal distance from the array. At the focal point, wavefronts from all of the input transducers arrive simultaneously and reinforce each other. The zero-order focal point is, therefore, at the center of curvature of the array, and each wavefront arriving at the focal point has traversed the same distance from an input transducer. A first-order focal point is laterally spaced from the zero-order focal point. Waves from two adjacent transducers still arrive at the first-order focal point in phase with each other, but their path lengths differ by one wavelength, or some other integral number of wavelengths. Now, if the frequency of the signal applied to the input array is changed, the first-order focal point is shifted laterally with respect to the zero-order focal point. If a wideband input signal is applied to the input array, the first-order focal point becomes a focal arc, each point on the arc representing a different input frequency. The output transducers are arrayed along the focal arc, and each is responsive to a narrow band of frequencies. This, then, is the basis for spectral analysis using SAW techniques.
Although the aforementioned copending application discloses a device of this kind that is generally satisfactory in most respects, there is still need for further improvement in the areas of frequency resolution and suppression of electromagnetic feedthrough. The present invention is directed to this need.